Polyphosphonitrilic chloride has been known for a long time (H. N. Stokes, Am. Chem. J. 19, 782 (1897)). It can be made by heating cyclic phosphonitrilic chloride. The polymer so made has been referred to as "inorganic rubber" because of its rubber-like qualities and insolubility in solvents. More recently it has been discovered that polyphosphonitrilic chloride could be made in the form of a substantially linear non cross-linked polymer (Allcock et al., J. Am. Chem. Soc., 87, 4216 (1965)). The non cross-linked polymer can be dissolved in solvents such as toluene, cyclohexane and tetrahydrofuran and while in solution reacted with reagents that replace the chlorine bonded to phosphorus with a broad range of substituent groups such as alkoxy, aryloxy and the like. These groups control the properties of the final polymer.
Once the desired substituents were in place, it became desirable to cure the polymer to convert it to a non-soluble more durable form with improved physical properties. It was found that this could be accomplished by adding peroxides and heating the composition to generate free radicals which caused the substituent groups to cross-link. Alternatively, the substituted polyphosphazenes could be sulfur-vulcanized the same as in the rubber industry if at least a portion of the substituent groups on phosphorus contained olefinic unsaturation. This was accomplished by adding sulfur and an accelerator plus other compounding ingredients and heating the blend to vulcanization temperature. Sulfur and peroxide cure are described in Kyker et al., U.S. Pat. No. 3,970,533 and in Cheng, U.S. Pat. No. 4,116,785.
Curing using these known methods requires heating the composition to fairly high temperatures to activate the curing agents. A need exists for a method of curing the high molecular weight non cross-linked polyphosphazenes at moderate temperatures.